Insecurity is known to be presently one of the chief preoccupations of our society. Certain public places and forms of public transport, such as the underground railway for example, are, of course, already equipped with alarm devices. However, the use of these conventional alarm devices poses problems for the users, which problems may be of a physical nature--inability to reach the nearest alarm device--or of a moral nature--the fear of attracting attention in the event of aggression.